In a housing of an outdoor unit of an air-conditioning apparatus, there are formed chambers such as a mechanical chamber storing a refrigerant circuit including a compressor, an electric component chamber storing electric components, and an air-sending chamber storing an air-sending device. In an existing outdoor unit of an air-conditioning apparatus, the air-sending chamber is divided from the mechanical chamber and the electric component chamber, but the mechanical chamber and the electric component chamber are not divided from each other. Consequently, sound generated in the mechanical chamber may leak to the outside of the outdoor unit from the air-sending chamber via the electric component chamber and cause noise. To address such noise, an air-conditioning apparatus is disclosed in which a compressor chamber and an electric component chamber are partitioned by a dividing plate to prevent noise generated by a compressor from leaking to the outside of an apparatus body (see Patent Literature 1). Further, an outdoor unit of an air-conditioning apparatus is disclosed in which a mechanical chamber partitioned from an air-sending device chamber is separated by an electric component box into a compressor chamber storing a compressor and an air passage chamber allowing communication between the air-sending device chamber and the outside (see Patent Literature 2). Patent Literature 2 discloses that the electric component box has one end surface contacting a dividing plate and is formed to have a shape contacting front and rear surfaces and a side surface of an outer box of the outdoor unit.